Polynuclear boron complexes

Studies other than preparative-type transformations have so far been limited to B-pyrazolylpyrazaboles. For example, the latter compounds have been found to react with R BX (where X is a readily leaving group) to form polynuclear boron spiro-cations (see Sect. V.A). It has also been shown that the pyrazolylpyrazaboles offer themselves as a class of neutral bidentate chelating ligands For example, the complexes H2B(pz)jB(pz)2ZnCl2 and Cl2Zn(pz)2B(pz)2B(pz)jZnCl2 were readily accessible by combination of the two reactants in an appropriate solvent. Obviously, a more detailed study of this feature is mandated. 

With the exception of polynuclear boron hydrides, cluster chemistry has developed mainly over the past 40 years. Prior to this a number of compormds that would now be classified as clusters had been but their nature was not recognized. It is perhaps interesting to ask the question of why cluster compounds, that is molecules containing metal-metal bonds, were considered prior to 1940 to be a relatively uncommon feature in inorganic chemistry The problem was in part associated with the dominance of organic chemistry coupled with the difficulties in the rapid determination of the structure of compounds of this type. The determination of the X-ray structure of relatively simple compounds could take months or years to accomplish. As stated above, for the s and p block elements the compounds were also generally found to be unstable to air and moisture, whilst for the d and f block the compounds were often very complex in structure and the stoichiometry difficult to assess. This led to a general lack of appreciation of the occurence and stability of metal-metal or element-element bonds. 

It should be clear from the discussion of Section 2 that all of these clusters can be unstable with respect to cluster degradation in the presence of Lewis bases with the formation of saturated fragment species (see Boron Hydrides and Polynuclear Organometallic Cluster Complexes). Thus, in the presence of an excess of phosphine, the reaction of [HFe3(CO)9(/u-H)2BH] changes from that shown in equation (10) to that shown in equation (12). The mononuclear boron and iron products demonstrate that both transition metal and main group element vertices are removed by base and the cluster is degraded. ... 

These and related methods of structure prediction depend upon the allocation of specific electron counts to different framework geometries and these applications have been very successful in assigning structures to cluster carbonyls and their derivatives. However, for the higher polynuclear carbonyls as the molecularity of the compounds increases the predictive power of the theories becomes less decisive in differentiating between alternative structures. In essence the Wade-Mingos approach assumes that the frontier orbitals of the complex primarily involve metal orbitals so that any variation in the electron occupation will be reffected in a structural change in the metal framework. The Wade theory also requires that the structure of the complexes are based on triangulated polyhedra as found for the boron hydrides. 

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